Poly(vinyl alcohol) homopolymer and copolymers, collectively referred to hereinafter as poly(vinyl alcohol) polymers, or by the abbreviation PVA or PVAs, are produced via hydrolysis of the corresponding ester of the homopolymer or the copolymer, i.e., poly(vinyl acetate), abbreviated herein as PVAC or PVACs. This hydrolysis of the polyester to the corresponding polyalcohol is typically conducted using either an acid or a base catalyst. The hydrolysis process may be catalyzed by strong acids or strong bases, and is known as hydrolysis, saponification, or alcoholysis, depending on the particular reaction conditions and the catalyst used. Reactions using base are generally preferred due to greater efficiency and reaction rate. Unfortunately, however, residual base from the hydrolysis of the ester tends to destabilize the PVA after isolation, and/or during subsequent processing and end use of poly(vinyl alcohol).
Bases are difficult to remove completely from the PVA since they tend to be strongly absorbed thereby. Base instability may be countered by neutralization of any base remaining before PVA isolation, typically using weak acids such as acetic acid to produce the corresponding salt of the base. The use of acetic acid to neutralize any residual base however results in the production of basic salts such as sodium acetate which are somewhat more amenable to washing out or otherwise separate from the poly(vinyl alcohol). Such salts themselves, while less destabilizing than the base itself, are still somewhat destabilizing to the PVA, particularly vis-a-vis recent more demanding uses of PVA.
Films comprising PVA are utilized in a variety of applications as laminates and the like where optical clarity is desirable. In addition, films utilized in such applications are preferably colorless, and maintain a clear-colorless appearance under a variety of conditions. Japanese Patent Number JP 11080483 is directed towards poly(vinyl alcohol) films comprising ammonium salts of acids having a pKa1 of less than or equal to 4.5, which when cast to form a film, show no discoloration (i.e., yellowing) after heat treatment at 200° C. for 10 min.
In addition, to maintain optical clarity, it is desirable to reduce or eliminate metallic residues which act as ash precursors. Such metallic residues include the basic salts resultant from the hydrolysis used to produce the PVA. Ash, as used here, refers to the oxide, hydroxide or related complex that results from heating metallic residues. Ash in the final PVA film results from thermal processing of the PVA in producing the film, or other object.
Ash precursors such as the basic salts in general, and sodium acetate (NaOAc) in particular, can amount to several percent of a PVA if steps are not taken to reduce the salt concentration. The presence of such residues is thought to reduce the otherwise exemplary properties of PVA in terms of clarity, yellowing, weatherability, physical properties, and the like. There is therefore a strong incentive to reduce the presence of various salts, in particular sodium acetate, to very low levels of less than about 0.2 weight percent in the PVA.
Various attempts directed to removal of NaOAc include Japanese Patent Number 45033425, directed to removal of residual NaOAc by contact of saponified poly-(vinyl alcohol) or copolymer thereof with NaOH over a strong acidic cation exchanger. In this example, an ethylene-vinyl acetate copolymer (40:60 mole ratio) was dissolved in MeOH, NaOH is added, and the PVAc is saponified to PVA. A portion of the saponified solution was stirred with 30 g acid-form of Amberlite IR-120 (a strong cation exchange resin gel), followed by separating the resin, concentrating to 40 weight % PVA, and drying at 80° C. for 1 hour to yield saponified copolymer containing 0.01% NaOAc.
Japanese Patent Number 02163130 is directed to removal of metal ions in non-electrolyte polymer aqueous solutions using ion exchange resins. In this example, aqueous poly(vinyl alcohol) was passed through a column filled with activated Diaion SK1B (a strong cation exchange resin gel) and Diaion SA10A (a strong anion exchange resin gel) to produce an aqueous solution containing <0.01 mg/L Na ion. A film of the PVA produced demonstrated an electrical conductivity of 3 μS/cm, compared to 330 μS/cm for the control.
U.S. Pat. Nos. 5,319,070 and 5,425,879 are directed to removal of metallic, ash-producing residues which remain in poly(vinyl alcohol) polymer after preparation from poly(vinyl acetate) by contacting a slurry of the poly(vinyl alcohol) polymer with ion-exchange resin particles, removal of the ion-exchange resin particles from the poly(vinyl alcohol) polymer particles by differential particle separation, and removing the liquid from the polymer and drying the polymer. The poly(vinyl alcohol) polymer remains in the solid state throughout, thus dissolution and re-precipitation of the PVA are avoided.
In the article entitled: Study of the Separation of Aqueous Mixtures of Poly(vinyl alcohol) and Sodium Acetate by Ultrafiltration, (Karamyan, D. R.; Voskanyan, P. S.; Movsesyan, R. A., Erevan. NII “Plastpolimer”, Yerevan, Armenia, Plasticheskie Massy (2001), (5), 39-40, Russian); an aqueous PVA solution was effectively purified from sodium acetate by ultrafiltration practically without polymer loss. Hollow fiber membranes can be used for solution separation, for suspension processing tubular ultrafiltration membranes with pore size ≦0.55 μm are preferable.
Czechoslovakian Patent Number 275541 is directed to purification of crude partially aqueous solutions of poly(vinyl alcohol) copolymers containing NaOAc and colored impurities by contacting with a cation and anion exchangers while stirring at 10-60° C.
Ion-exchange resins (IERs) are known in the removal of metal residues from PVA, as disclosed in U.S. Pat. No. 2,940,948. In the examples, an aqueous solution of the PVA is mixed with a strong cationic exchange resin gel, which is then removed by filtration.
Accordingly, removal of NaOAc and other basic salts from PVA is an area of interest in the art. However, a need exists for a continuous process for removing basic salts from aqueous, homogeneous solutions of PVA. Furthermore, in view of the ever increasing uses of PVA which require optically pure, non-yellowing PVA films, a continuous process of producing PVA which produces a non-yellowing PVA film is also desirable.